T2111.hs:1:16:
    Warning: -XRecursiveDo is deprecated: use -XDoRec or pragma {-# LANGUAGE DoRec #-} instead

T2111.hs:6:23:
    The arrow command
      undefined -< x
    was found where an expression was expected
    In a stmt of a 'do' block: x <- undefined -< x
    In a stmt of a 'do' block: rec {x <- undefined -< x}
    In the expression:
      do { rec {x <- undefined -< x};
           undefined -< x }